This application claims the priority of Korean Patent Application No. 2002-51972, filed on Aug. 30, 2002, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an optical disc system, and more particularly, to a data slicer, which converts an analog signal extracted from an optical disc into a digital signal, and a data slicing method for an optical disc reproducer.
2. Description of the Related Art
FIG. 1 is a schematic block view of a standard optical disc reproducer. As shown in FIG. 1, the optical disc reproducer includes an RF unit 110 for receiving a signal reflected from an optical disc in order to output an analog signal, a data slicer 120 for receiving the analog signal outputted from the RF unit 110 in order to estimate a median value of the analog signal and convert the analog signal into a digital signal using the median value, and a digital signal processing unit 130 for processing the digital signal output from the data slicer 120 in order to restore information from the signal. In addition to the above components, the optical disc reproducer further includes a motor unit 140 for mechanically controlling the optical disc, among other functions.
FIGS. 2 and 3 are circuit views of one example of a data slicer according to the prior art.
A data slicer shown in FIG. 2 includes a comparator 210, a phase locked loop (PLL) 220, a low pass filter 230, and an amplifier 240.
The comparator 210 compares an analog input signal AIN input from the RF unit 110 (refer to FIG. 1) with a predetermined feedback signal FB and outputs a digital signal EFMCOMP based on the comparison result. The PLL 220 receives the digital signal EFMCOMP and generates a channel clock signal PLCK having a channel period T. The low pass filter 230 low-pass filters the digital signal EFMCOMP, and the amplifier 240 amplifies the low-pass filtered digital signal to generate the feedback signal FB that is input to a negative terminal of the comparator 210.
The low pass filter 230 functions as an integrator unit. That is, the median value of the analog input signal AIN is estimated by integrating the digital signal EFMCOMP. The estimated median value is fed back to the comparator 210 through the amplifier 240 and is compared with the analog input signal AIN, and thus, the estimated median value is used as a slice level. The slice level is referred to as a base level for converting the analog input signal AIN into a digital signal such as a high level (1) and a low level (0). The slice level must be set at a center of an eye pattern of the analog input signal AIN. If the slice level deviates at the center of the eye pattern, an error is generated in the pulse widths at the high level and the low level when the analog input signal AIN is converted into the digital signal EFMCOMP so that there is a high possibility of generating a data error.
An enhanced data slicer, such as the one shown in FIG. 3, further includes a charge pump 250 in addition to the components of the data slicer shown in FIG. 2.
In this embodiment, if the digital signal EFMCOMP is at a high level, a switch SW1 of the charge pump 250 is turned “OFF” and a switch SW2 of the charge pump 250 is turned “ON.” In this case, current flows from an output node of the charge pump 250 to ground so that a voltage charged to a capacitor CP is reduced, and thus, the signal level input to the low pass filter 230 is lowered. If the digital signal EFMCOMP is at a low level, the switch SW1 is turned “ON” and the switch SW2 is turned “OFF.” In this case, current is supplied from the power supply voltage and the capacitor CP is charged with the voltage so that the signal level input to the low pass filter 230 is raised.
Thereafter, since the charge pump 250 lowers the peak-to-peak level of the digital signal EFMCOMP, the charge pump 250 functions to reduce a design feature, particularly, the bandwidth of the low pass filter 230 connected to the rear end of the charge pump 250.
However, the data slicer of the prior art shown in FIGS. 2 and 3 has a high possibility of generating a data error, in the case where the eye pattern of the analog input signal ALN is asymmetrical.
FIG. 4 is a waveform diagram of an analog input signal. Generally, a waveform of an analog input signal is referred to as an eye pattern. In FIG. 4, 11T is referred to as a signal have a relatively long swing width, and 3T is referred to as a signal having a relatively small swing width.
A compact disc (to be referred to hereinafter as a CD) system or a digital versatile disc (to be referred to hereinafter as a DVD) system modulates information data using an EFM (Eight-to-Fourteen Modulation) and stores the modulated data. In the case of the CD, an EFM signal is a pulse signal having a period of 3T to 11T (T is a channel clock period), and in the case of the DVD, an EFM signal is a pulse signal having a period of 3T to 14T. Thus, when it is considered that the CD is the base, an analog input signal reproduced from an optical disc corresponds to any one of the signals having a period of 3T to 11T.
In FIG. 4, (a) presents the case where the center level of an 11T signal is identical with the center level of a 3T signal. FIG. 4(b) presents the case where the center level of the 3T signal has a positive value with respect to the center level of the 11T signal. That is, FIG. 4(b) shows the case where positive asymmetry occurs. On the other hand, FIG. 4(c) presents the case where the center level of the 3T signal has a negative value with respect to the center level of the 11T signal. That is, FIG. 4(c) shows the case where negative asymmetry occurs.
In the case where the amount of recording power is inappropriate when data is recorded to the disc, or in the case where pit asymmetry occurs during manufacturing the disc, etc., asymmetry in the analog input signal is generated as in FIGS. 4(b) and 4(c).
The data slicers according to the prior art shown in FIGS. 2 and 3 estimate the median values of all of the 3T signals through the 11T signals. Thus, median values of the signals with the relative long pulse width (11T signal or signals near to the 11T signal) greatly affect the median values of all of the signals.
In a case where an analog input signal is asymmetrical as in FIGS. 4(b) and (c), and if the median value of the 11T signal is used as a slice level and the median value of the 11T signal is applied to the 3T signal, a large number of errors are generated in the pulse widths at the high level and low level of the 3T signal. Note that an error is not generated in restoring data only in the case where the analog input signal is symmetrical. However, as shown in FIGS. 4(b) and (c), when it is considered that the 11T signal is the base, the pulse width at the low level of the 3T signal is longer than that of the high level, or the pulse width at the high level of the 3T signal is longer than that of the low level. As a result, there is a high possibility of generating a data error in restoring data.
Thus, since median values of signals with a relative small pulse width (3T signal or signals near to the 3T signal) cannot be accurately estimated in the prior art, it is difficult to restore data accurately, whereby there is a high possibility of generating a data error.